Nitride semiconductors such as GaN, AlN, and InN, materials including mixed crystals thereof, and the like have a wide band gap and therefore are used in high-power electronic devices, short-wavelength light-emitting devices, and the like. As for the high-power electronic devices, techniques related to field-effect transistors (FETs), particularly high-electron mobility transistors (HEMTs), are under development. HEMTs including such nitride semiconductors enable large-current, high-voltage, low-on-resistance operation and therefore are used in high-power, high-efficiency amplifiers, high-power switching devices, and the like.
An HEMT made of GaN or the like is formed in such a manner that a buffer layer is formed on a substrate of a semiconductor or the like, an electron travel layer of i-GaN or the like is epitaxially grown on the buffer layer by metal-organic vapor phase epitaxy (MOVPE) or the like. In particular, as illustrated in FIG. 1, a buffer layer 921, an electron travel layer 923 (923a illustrates a two dimensional electron gas.), a spacer layer 924, an electron supply layer 925, and a capping layer 926 are formed in series on a substrate 910 and a gate electrode 931, a source electrode 932, and a drain electrode 933 are formed on the capping layer 926. An HEMT including a configuration illustrated in FIG. 1 has a problem that since the buffer layer 921 has low resistance, a current flows through the buffer layer 921 and therefore a leakage current increases.
Therefore, an HEMT including a configuration in which a high-resistance layer 922 is disposed on a buffer layer 921 as illustrated in FIG. 2A is under investigation. In particular, the buffer layer 921, the high-resistance layer 922, an electron travel layer 923, a spacer layer 924, an electron supply layer 925, and a capping layer 926 are arranged in series on a substrate 910 and a gate electrode 931, a source electrode 932, and a drain electrode 933 are disposed on the capping layer 926. In the HEMT including the configuration illustrated in FIG. 2A, the high-resistance layer 922, which has high resistance, can be formed by doping GaN with a transition metal such as iron (Fe), thereby enabling increased insulation.
However, gas including containing the transition metal, such as Fe, used for doping remains in a growth furnace of an MOVPE system used to form the high-resistance layer 922 and therefore the transition metal, such as Fe, enters the electron travel layer 923, which is formed subsequently to the formation of the high-resistance layer 922. In particular, Fe enters the electron travel layer 923 to form a high-Fe concentration region as illustrated in the distribution of Fe in the high-resistance layer 922 and the electron travel layer 923 in FIG. 2B. The electron travel layer 923 is made of i-GaN or the like. The entrance of the transition metal, such as Fe, into the electron travel layer 923 reduces the mobility of electrons in the electron travel layer 923 to cause an increase in on-resistance. The increase in thickness of the electron travel layer 923 may be taken to reduce the on-resistance of the electron travel layer 923. However, the increase in thickness of the electron travel layer 923 causes an increase in leakage current.
Japanese Laid-open Patent Publication No. 2002-359256 is an example of related art.